Spatial distribution of random velocity inhomogeneities in the western part of Nankai subduction zone

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Author(s): Takahashi, T.; Obana, K.; Yamamoto, Y.; Nakanishi, A.; Kodaira, S.; Kaneda, Y.
Author Affiliation(s): Primary:
Japan Agency for Marine-Earth Sciences and Technology, Institute for Research on Earth Evolution, Yokohama, Japan
Volume Title: AGU 2011 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2011; American Geophysical Union 2011 fall meeting, San Francisco, CA, Dec. 5-9, 2011. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: In the Nankai trough, there are three seismogenic zones of megathrust earthquakes (Tokai, Tonankai and Nankai earthquakes). Lithospheric structures in and around these seismogenic zones are important for the studies on mutual interactions and synchronization of their fault ruptures. Recent studies on seismic wave scattering at high frequencies (>1Hz) make it possible to estimate 3D distributions of random inhomogeneities (or scattering coefficient) in the lithosphere, and clarified that random inhomogeneity is one of the important medium properties related to microseismicity and damaged structure near the fault zone [Asano & Hasegawa, 2004; Takahashi et al. 2009]. This study estimates the spatial distribution of the power spectral density function (PSDF) of random inhomogeneities the western part of Nankai subduction zone, and examines the relations with crustal velocity structure and seismic activity. Seismic waveform data used in this study are those recorded at seismic stations of Hi-net & F-net operated by NIED, and 160 ocean bottom seismographs (OBSs) deployed at Hyuga-nada region from Dec. 2008 to Jan. 2009. This OBS observation was conducted by JAMSTEC as a part of "Research Concerning Interaction Between the Tokai, Tonankai and Nankai Earthquakes" funded by Ministry of Education, Culture, Sports, Science and Technology, Japan. Spatial distribution of random inhomogeneities is estimated by the inversion analysis of the peak delay time of small earthquakes [Takahashi et al. 2009], where the peak delay time is defined as the time lag from the S-wave onset to its maximal amplitude arrival. We assumed the von Karman type functional form for the PSDF. Peak delay times are measured from root mean squared envelopes at 4-8 Hz, 8-16 Hz and 16-32 Hz. Inversion result can be summarized as follows. Random inhomogeneities beneath the Quaternary volcanoes are characterized by strong inhomogeneities at small spatial scale (∼ a few hundreds meter) and weak spectral gradient. Those in the Hyuga-nada region are characterized by the strong inhomogeneities at large spatial wavelength and steep spectral gradient. Random inhomogeneities in the Hyuga-nada region are similar with those in the frontal arc high in northern Izu-Bonin arc, which is thought to be a remnant arc that is presently inactive [Takahashi et al. 2011]. This coincidence implies the existence of subducted Kyushu-Palau ridge in this anomaly of random inhomogeneities, which is also suggested by the seismic refraction survey in this region [Nakanishi et al. 2010 AGU Fall Mtg.]. Source rupture areas of large earthquakes (M>6) in Hyuga-nada regions tend to locate around this anomaly of inhomogeneities. We may say that this anomalously inhomogeneous region is a structural factor affecting the seismic activity in Hyuga-nada region.
Year of Publication: 2011
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 16 Structural Geology; 19 Geophysics, Seismology; Anomalies; Crust; Detection; Earthquakes; Elastic waves; Faults; Geophysical methods; Heterogeneous materials; Integrated Ocean Drilling Program; Lithosphere; NanTroSEIZE; Nankai Trough; North Pacific; Northwest Pacific; Ocean bottom seismographs; Pacific Ocean; Rupture; Seismic methods; Seismic networks; Seismic waves; Seismic zoning; Seismographs; Subduction zones; Velocity structure; Wavelength; West Pacific
Record ID: 2015099583
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by, and/or abstract, Copyright, American Geophysical Union, Washington, DC, United States

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